Unsaturation polyether polyols

The anionic polymerisation of PO is in fact a competition of two simultaneous reactions the propagation reaction (R ) and the transfer reaction (Rtr). An interesting way to obtain it directly from the synthesis of low unsaturated polyether polyols, is to accelerate selectively the propagation reaction while the transfer reaction remains unchanged or lower. It is well known that low unsaturation of polyether polyols represents a low monol content and leads to better physico-mechanical properties in the resulting PU, because in the PU chemistry the monol (a monofunctional compound) is a chain stopper, (i.e., it stops the MW increase). 

A very interesting group of random copolyethers is obtained by anionic copolymerisation of EO (a highly hydrophilic monomer) with BO (a highly hydrophobic monomer). Because EO does not isomerise to double bond structures and BO has a much lower tendency to isomerise to allyl structures than PO (see Chapter 12.2), the BO-EO copolyethers have a very low unsaturation level compared to PO homopolymers or even PO-EO copolymers [82]. This variant of polyether polyols synthesis in the form of BO-EO copolymers is a very interesting way to obtain low unsaturation polyether polyols directly from synthesis. Another group of low unsaturation polyether polyols, obtained directly from synthesis, are the tetrahydrofuran (THF)-EO and THF-PO copolymers synthesised with cationic catalysts (see Chapter 7.3). 

GENERAL PROCEDURE FOR THE PREPARATION OF UNSATURATED POLYESTER RESIN FROM A POLYETHER POLYOL... 

Polyether polyols are commercially available in a molecular weight range similar to those of unsaturated polyesters (a few thousands). 

Thus, the rate of transfer with a PO monomer with a much higher activation energy varies more with the temperature than the PO propagation reaction. As an immediate consequence, by the decreasing the polymerisation temperature from 110-120 °C to 80 °C [69], polyether polyols with much lower unsaturation are obtained. In order to get convenient reaction rates, the catalyst concentration was increased. Table 4.3 shows the variation of propagation constant Kp of PO anionic polymerisation as a function of temperature. 

Tables 4.9-4.14 show some general characteristics of polyether polyol PO-EO block copolymers, such as MW, hydroxyl number, functionality, viscosity and colour, but also some other characteristics such as unsaturation, EO content, and potassium and sodium content which are specific for polyether polyols.

Unsaturation (standard test methods ASTMD4671 [111] and ISO 17710 [112] represents the amount of terminal double bonds in polyether polyols. One usual method is chemical determination of the double bond content based on the quantitative reaction of mercuric acetate with double bonds in methanol. The reactions involved are ... 

The unsaturation is expressed in milliequivalents of double bonds per one gram of polyether (mequiv/g). Recent methods for determination of terminal unsaturation in polyether polyols are based on NMR and 13C NMR spectroscopic methods [88]. 

Phosphazenium hydroxides or alcoholates are very active catalysts for PO and EO anionic polymerisation initiated by glycerol, DPG or other starters, at lower temperatures of around 80-100 °C, giving colourless polyether polyols with very low unsaturation (around 0.02 mequiv/g at higher MW 5000-6000 daltons), which represents a very good performance in the synthesis of polyether polyols for PU. 

DMC catalysts are considered to be the ones that perform best at this time for PO polymerisation initiated by hydroxyl groups. Bayer developed the first continuous process, with a very high productivity, for the synthesis of polyether polyols with DMC catalysts (IMPACT Catalyst Technology). In a short and simple production cycle, a large variety of polyether diols of very low unsaturation for elastomers, sealants, coatings and low monol content polyether triols destined for flexible polyurethane foams are obtained. This is one of the best developments in the last few years in the field of polyether polyol synthesis [2],... 

A very convenient method to obtain a macromer (structure 6.13) is by using an unsaturated isocyanate [50] or by using some accessible raw materials, by the reaction of a hydroxy alkyl acrylate or methacrylate with one -NCO group of a diisocyanate for example, 2,4 toluene diisocyanate (TDI) and to react the remaining -NCO group with the terminal hydroxyl group of a polyether polyol ... 

Reaction 11.4 is used to decrease the unsaturation of polyether polyols simultaneously with the functionality increase. Thus, by introducing a polyether polyol with high unsaturation (0.07-0.09 mequiv/g), a low molecular weight polysiloxane compound, having 2-3 Si-H groups/mol, together with a platinum catalyst, the polyether monol present in the polyether (in fact allyl ether based polyethers) is added to the polysiloxane compound and the monol is transformed into a diol or into a triol (reaction 11.5). 

Propylene oxide, made by reacting propylene with chlorine to form propylene chloro-hydrin which is then dehydrochlorinated with caustic soda or lime (Eqs. 3-5), is used in the production of polyether polyols used for producing urethane foam. It also finds use in propylene glycol for making unsaturated polyester resins and in the pharmaceutical and food industries. Epichlorohydrin (EPI), formed by chlorination of propylene to allyl chloride and then dehydrochlorination (Eqs. 6 and 7), is used to make epoxy resins for producing laminates, fiber-reinforced composites, protective coatings, and adhesives. 

Excellent PO-EO block copolyether polyols with terminal poly[EO] block, are formed by the addition, to the intermediate propoxylated polyether obtained with DMC catalysts, of an anionic catalyst (KOH or potassium alcoholates) followed by the addition of EO by classical technology, via an anionic mechanism. By this relatively complicated route, it is possible to obtain PO-EO block copolymers with high primary hydroxyl content and very low unsaturation. 

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